The present invention relates to adaptive equalizers, and more particularly, to an adaptive cross-polarization equalizer that provides for cancellation of an interfering signal due to low cross-polarization isolation caused by rain on high data rate communication links.
With the increasing utilization of radio frequency communications, the allocatable radio frequency spectrum is rapidly becoming limited. Commercial allocations are encroaching on frequencies which to date have been available for broadband communications. In the future, it will be difficult to provide contiguous RF allocations of several gigahertz for multi-gigahertz communications systems. Therefore, the radio frequency spectrum must be more efficiently managed and utilized. Frequency reuse, using orthogonal polarizations, is one approach to greater efficiency.
Adaptive baseband cancellation architectures have heretofore been developed in order to increase the available radio frequency spectrum utilization, but without complete success. These types of architectures typically use "four rail" adaptive baseband transversal equalizers. These architectures require computation of the correlation of the interference on the I and Q signals of each channel with the I and Q signals of the other channel as well as the I and Q signals within each channel. These are very complex architectures and are tied to one modulation and data rate.
The present invention addresses the problem of depolarization in communications systems. Depolarization introduces an interfering signal from the cross-polarized signal into the co-polarized signal. The primary source of interfering signals that cause depolarization is rain. To optimize system performance, the effects of the interfering signals must be minimized. Available field test data and analysis to date indicate that cross-polarization interference due to rain has a fluctuating amplitude and phase shift but that the depolarizing phenomena is nondispersive. The rate of fluctuation is estimated to be less than 1 Hz. Furthermore, due to the dispersive nature of filters in downconverters and demodulators used in adaptive equalizers and to avoid having to match delays between the two receivers, it is desirable to perform cross-polarization cancellation as close to the front end of the microwave chain as possible.
For the purposes of reference, U.S. Pat. No. 5,157,697 issued to Anvaru describes a system that suppresses crosstalk between orthogonal channels by subtracting a portion of the signal of one channel from the other as controlled by correlation factors. U.S. Pat. No. 4,466,132 issued to Namiki describes a system that eliminates crosstalk between two mutually orthogonal cross-polarized channels. The following references describe systems that are generally similar to the Namiki patent: U.S. Pat. No. 4,112,370 issued to Monson; U.S. Pat. No. 4,438,530 issued to Steinberger, U.S. Pat. No. 4,479,258 issued to Namiki; U.S. Pat. No. 4,637,067 issued to Steinberger; and U.S. Pat. No. 4,688,235 issued to Tahara et al. U.S. Pat. No. 3,735,266 issued to Amitay and U.S. Pat. No. 4,090,137 issued to Soma et al describe systems that reduce or minimize crosstalk between cross-polarized channels utilizing pilot signals to indicate the level of crosstalk.
Therefore, it is an objective of the present invention to provide for an adaptive cross-polarization equalizer that provides for cancellation of an interfering signal due to low cross-polarization isolation caused by rain on high data rate communication links.